The present invention generally relates to amplifier circuits, and more particularly to an amplifier circuit provided with a variable gain amplifier a gain of which is limited when an input signal level is excessively large.
Normally, a linear amplifier circuit is used as an amplifier circuit for amplifying an audio signal, and amplitudes of an output signal V.sub.out and an input signal V.sub.in of the amplifier circuit have a predetermined proportional relationship as shown in FIG. 1. This predetermined proportional relationship is maintained until a transistor at an output stage of the amplifier circuit saturates. However, in a saturation region of the output stage transistor where the amplitude of the input signal V.sub.in is greater than V.sub.s, the waveform of the input signal V.sub.in does not coincide with the waveform of the output signal V.sub.out. In other words, the proportional relationship differs about the input signal V.sub.in of V.sub.s as indicated by a solid line in FIG.1. In the saturation region of the output stage transistor, there is a notable deterioration in a distortion factor due to higher harmonic components as may be seen from FIG. 2.
A conventional amplifier circuit has a variable amplifier for amplifying an input signal, a constant gain amplifier for amplifying an output signal of the variable gain amplifier and for outputting an output signal V.sub.out, and a level detection circuit for detecting an output signal level of the variable gain amplifier. The level detection circuit variably controls the variable gain amplifier so as to limit the level of the output signal V.sub.out to V.sub.Lo as indicated by a phantom line in FIG.1 when the output signal of the variable gain amplifier is greater than a predetermined level, that is, when the level of the input signal of the constant gain amplifier is greater than V.sub.Li and excessively large. Such a conventional amplifier circuit is known as an automatic gain control (AGC) circuit or an automatic level control (ALC) circuit.
In the conventional amplifier circuit, the level of the output signal V.sub.out is limited to V.sub.Lo which is constant when the input signal V.sub.in is greater than V.sub.Li. As a result, there is a problem in that no signal amplification can be carried out in a vicinity of a maximum tolerable input signal level of the constant gain amplifier. In addition, there is also a problem in that a circuit operation of the constant gain amplifier becomes unstable when a change occurs in a power source voltage.
In other words, the output signal of the constant gain amplifier is clipped at a clipping level when the level of the input signal is greater than the maximum tolerable input signal level of the constant gain amplifier, and in a case where the input signal is an audio signal the sound is distorted. For this reason, the conventional amplifier circuit reduces the gain of the variable gain amplifier by a feedback loop when the output signal of the constant gain amplifier is greater than a reference level. In this case, it is desirable that the reference level is slightly lower than the clipping level. However, the clipping level of the constant gain amplifier differs depending on the amplifier circuit (that is, an integrated circuit of the amplifier circuit) and also changes depending on a change in the power source voltage. When the reference level is high, there is no feedback by the feedback loop when the clipping level is low thereby lacking in the flexibility of use of the amplifier circuit.